Cysteine prodrugs are useful for protecting animals against radiation-related deaths, effects caused by radiation poisoning, and acetaminophen-induced hepatotoxicity (Lenarczyk et al., Radiation Res., 2003, 160, 579-583; Roberts et al., J. Med. Chem., 1987, 30, 1891). Cysteine prodrugs, such as, 2(R,S)-D-ribo-(1′,2′,3′,4′-tetrahydroxybutyl)thiazolidine-4(R)-carboxylic acid (RibCys), have been used in methods to enhance delivery of glutathione and ATP levels in cells (see, U.S. Patent Publication No. 2009-0042822). Although there have been uses described for other cysteine prodrugs, such as N-acetylcysteine (NAC), compositions to deliver cysteine to an animal have been limited due to, for example, instability and/or the reactivity of the exposed sulfhydryl (SH) group of cysteine. Thus, there is a long-felt need to solve the problem of how to deliver cysteine to an animal in a convenient form. The present invention solves this need as well as others.